1. Field of the Invention
The present invention relates in general to an analog-to-digital converter. In particular, the present invention relates to a current-mode chain-reaction analog-to-digital converter.
2. Description of the Related Art
In many mixed-signal systems, analog-to-digital converters (ADC) are required for interfacing analog signals to digital circuits. The requirement is usually to integrate these ADCs with digital signal processors (DSP) in a low-cost CMOS technology. However, due to reliability issues, the supply voltages for advanced CMOS processes have been continuously reduced and will be further reduced to 1.5V and below in the near future. As a result, ADCs integrated with a DSP are required to operate in the same range of supply voltage. However, designing an ADC to operate at such a low supply voltage presents a great challenge, due to the fact that the threshold voltage of MOSFET devices are relatively high for the given supply voltage ranges even for future CMOS processes. To address this challenge, different techniques have been used to realize low-voltage ADCs including the use of specialized processes that provide low-threshold devices, bootstrap techniques, and switched-op-amp techniques. For the 0.25 um CMOS process, the ADC of the chip has a poor transforming characteristic when operating at 1.5V˜2.5V. Thus, a voltage converter is required for the DAC circuit to raise the operation voltage to 3.3V or higher. However, the chip size and the cost of the circuit increase, and noise, short-channel effects and chip latching also occurs.